This invention relates to a method for preparation of high molecular weight polyester material, such as pellets or coarse powder, by the solid state crystallization and polymerization or polycondensation of polyesters of lower molecular weight.
The common commercial practice for producing high molecular weight polyester in the solid state involves thermal processing in an inert gas atmosphere or under vacuum at temperatures below the material melting point. High crystallization and/or polymerization rates can be achieved at substantially elevated temperature, but in commercial practice, the reaction temperature is usually restricted by the relatively low sticking temperature of polyester particles. The tendency of the polyester particles to agglomerate when the sticking temperature is reached requires special efforts and technical means to prevent the undesirable effects of such agglomeration and caking in the crystallization/polymerization apparatus during the critical sticky phase.
In order to avoid agglomeration due to stickiness, the prior art has subjected the granulate to continuous vigorous mechanical or aerodynamical agitation. Methods based on mechanical agitation are disclosed, for example, in U.S. Pat. Nos. 4,151,578 and 4,223,128. Methods based on the fluidized bed concept are described in German Patent DE-OS 1,467,213 and in U.S. Pat. No. 5,090,134.
Methods based on mechanical agitation have been utilized in systems incorporating an indirect (conductive) heat supply concept and therefore have limited efficiency in terms of heat transfer and ability to provide an optimal temperature control. These proposals have met with a limited degree of success because it is necessary to employ relatively low processing temperatures thereby restricting the rate of crystallization and/or polymerization. In addition, vigorous agitation of the polyester granules during the thermal processing has a negative effect on the product quality in terms of fines generation and possible deformation of the polyester pellets.
Methods based on aerodynamical agitation (or the fluidized bed concept) have the essential disadvantage of requiring very large quantities of inert gas and in many cases cannot be economically justified due to very high energy consumption and operating cost.
In one attempted solution to solve the sticking problem as described in U.S. Pat. No. 3,014,011, the amorphous polyester pellets are treated with water, saturated steam or organic solvents at temperatures up to 110.degree. C. to effect at least partial crystallization of the granules. The granules are dried at a temperature of about 160.degree. C. to 170.degree. C. while holding movement of the granules to a minimum.
The process according to U.S. Pat. No. 3,014,011 is limited considerably by the boiling temperature of the liquid medium under atmospheric pressure conditions. For example, water boils at 100.degree. C. and for the organic solvent claimed in this proposal (toluene, benzene, nitromethane and acetone) the toluene has the highest boiling point (110.6.degree. C.) under atmospheric pressure. The process temperature is also quite restricted by the fact that crystallization of the polyester is carried out without forced agitation, i.e., holding movement of the granules to a minimum. Accordingly, this method is not subject to being commercialized at such low operating temperatures in the course of solid state crystallization and/or polymerization of polyester material.